Multifunctional SnO2 QDs/MXene Heterostructures as Laminar Interlayers for Improved Polysulfide Conversion and Lithium Plating Behavior

Highlights The interfacing between SnO2 and MXene alters electronic structures, shifting the d-band center in transition metals, enhancing catalytic efficiency by reducing electron filling in antibonding orbitals. A binder-free, ultrathin, laminar heterostructured interlayer on polypropylene separator is demonstrated. The ionic sieving mechanism and efficient adsorption–catalysis process enable deeper charge/discharge cycle and improved stability. The improved catalytic conversion and suppressed lithium dendrites formation enable a high loading of 7.5 mg cm−2 and an initial area capacity of 7.6 mAh cm−2. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01446-w.


Introduction
Lithium-sulfur (Li-S) batteries, with their high theoretical specific capacity (1675 mAh g −1 ), eco-friendliness, and abundant availability of S, hold great promise as nextgeneration energy storage technology [1][2][3].However, their commercialization faces challenges pertaining to both cathode and anode.In particular, the S cathode exhibits inherent limitations such as sluggish redox kinetics, dissolution of intermediate lithium polysulfides (LiPSs), and the shuttle effect, which result in limited discharge depth and poor cyclic stability [4,5].On the other hand, safety concerns arise from the dendrite growth observed at the Li anode [6,7] and/or from Li corrosion [8,9].Addressing these challenges is crucial for the successful commercialization of Li-S batteries and unlocking their full potential as advanced energy storage systems.
The design of an efficient separator interlayer has been proved effective in mitigating the shuttle effect and suppressing the formation of Li dendrites [10][11][12].Compared with sulfur cathode modification, the separator strategy allows limited dissolution of polysulfides but restricts their migration to the cathode side.By tuning the permeability of the separator to selectively transport Li ions while blocking anions (i.e., polysulfides), dissolved LiPSs are retained on the cathode side, thus prevent their shuttling [13,14].In particular, by designing an elaborated interlayer with strong adsorption and high catalytic activity, the charge transfer and LiPSs redox kinetics can be significantly improved [15,16].In addition, interlayers featuring lithiophilic sites and a high Young's modulus on the anode side are harnessed to homogenize Li ion flux and restrict the growth of Li dendrites during the charging process [17,18].Recent studies have demonstrated the effectiveness of dual-function separator interlayers with both LiPSs redox kinetics enhancement and Li dendrite inhibition [19,20].These interlayers are typically composed of carbon or metal compounds with diverse structures, such as porous carbon networks [21], hollow structures [22,23], and hierarchical nanosheets [24][25][26].It is worth noting that, these interlayer frameworks show high porosity and exhibit ~ µm thickness.However, excessive porosity of the interlayer is undesirable as it requires more electrolyte to fill the pores, thereby compromising the overall cell energy density [27].Besides, the presence of inactive components, i.e., the separator interlayer, often referred to as "dead weight," is an inevitable consequence that should also be taken into consideration [28].Alternatively, the laminar structure, formed by the self-stacking of two-dimensional (2D) nanosheets, provides selective channels within the empty voids among neighboring nanosheets in both in-plane and out-of-plane directions, resulting in a dense framework and preventing the formation of large pores [29,30].Hence, an ultrathin laminar interlayer, combining permeability and selectivity, while featuring low porosity and negligible weight, is of significant promise.
MXene is an emerging 2D material with high potential in energy conversion and storage applications [31,32].The material is renowned for its exceptional conductivity (in Ti 3 C 2 T x MXene), atomic-scale thickness, and abundant terminal functional groups such as -OH, -O, and -F [33].These functional groups play a crucial role in anchoring LiPSs and accelerating their catalytic conversion in Li-S batteries [34].However, since the functional groups provide the main adsorption sites for LiPSs, the binding strength is relatively weak, resulting in a limited catalytic activity [35].Tuning the electronic states of MXene can potentially promote the intrinsic catalytic properties from the inert MXene surfaces [36,37].Modulating the electronic structure through constructing a Mott-Schottky heterostructure represents an effective method, thereby inducing an oriented and strong internal electric field [38,39].The altered electronic structure on both sides of the heterojunction leads to strong chemical adsorption and highly-efficient catalytic effects for LiPSs [40,41].For instance, Sun et al. fabricated a Mott-Schottky heterostructure by encapsulating metallic Co nanoparticles in N-doped carbon [42].They revealed that the redistribution of charge at the heterojunction can propel Li ion mobility, enhance LiPSs immobilization, and reduce the reaction energy barrier.Therefore, engineering heterostructures with a rational structural design holds promise as an effective strategy for achieving high-performance Li-S batteries.
Herein, we present the design of an ultrathin and laminar separator interlayer for Li-S batteries, utilizing a 0D-2D SnO 2 quantum dots (QDs)/MXene heterostructure (SnO 2 @ MX).Such novel heterostructure combines the highly polar SnO 2 , conductive MXene, and active heterojunctions, synergistically enabling strong anchoring of LiPSs, rapid electron/ion transportation, and efficient catalytic conversion.By introducing the optimized SnO 2 @MX separator 1 3 interlayer, we successfully enhance the conversion kinetics while effectively suppressing the shuttle effect and inhibiting Li dendrite growth.The fabricated Li-S cell with the SnO 2 @MX separator interlayer demonstrates excellent electrochemical performances.It achieves a high initial capacity of > 1400 mAh g −1 at 0.05 °C, 845 mAh g −1 at 2 °C, and a low-capacity decay of only 0.052% per cycle over 500 cycles.Furthermore, under a high sulfur loading of 7.5 mg cm −2 , a high initial areal capacity of 7.6 mAh cm −2 with a decent stability can be obtained.These findings pave new ways for further advancements in the field of Li-S batteries and highlight the potential of heterostructured catalysts for high-performance energy storage applications.

Synthesis of MXene
The Ti 3 AlC 2 MAX (hexagonal carbides and nitrides with general formula M n+1 AX n ) phase (Laizhou Kai Kai Ceramic Materials Co., Ltd.) was selectively etched using a minimally intensive layer delamination (MILD) synthesis method to produce Ti 3 C 2 T x MXene.Typically, 3.2 g of lithium fluoride (LiF, Sigma-Aldrich) was dissolved in 40 mL, 9 M hydrochloric acid (HCl, 37%, VWR) as the etching solution.Subsequently, 2 g of MAX phase was slowly added to above etching solution under vigorously stirring.The etching process was conducted for 48 h at 50 °C.After etching, the mixture was transferred to centrifuge tubes and centrifuged at 1500 rcf for 5 min.The supernatant was decanted, and the sediment was washed with 40 mL of ultrapure water.The washing step was repeated 5 times, each time centrifuging for 5 min at 1500 rcf, until the pH of the supernatant reached approximately 6.The obtained suspension was vigorously shaken using a vortex machine for 30 min.Then, the mixture was subjected to ultrasound treatment for 1 h under an ice bath with Ar bubbling to further delaminate into few-layered or single-layered MXene.After centrifuging the mixture at 1500 rcf for 30 min, the fully delaminated MXene nanosheets were obtained in the supernatant.The supernatant was then further centrifuged at 15,000 rcf for another 30 min to collect the MXene nanosheets.

Preparation of SnO 2 @MX-PP and MX-PP
The obtained SnO 2 @MX was thoroughly washed with N-methyl-2-pyrrolidone (NMP, Merck) with the aid of a vortex mixer and centrifuged at 15,000 rcf for 3 times.The resulting sediment was SnO 2 @MX mixed with NMP solvent.Then, the solvent exchanged SnO 2 @MX was slightly diluted with NMP and ground to form a uniform paste, which was subsequently blade coated onto one side of the PP (polypropylene) separator.The coated separator was then vacuum dried overnight at 60 °C.According to the necessity, the same process could be repeated for the modification of the other side after the first side was completely dried.Finally, the SnO 2 @MX-modified PP separator was punched into wafers with a diameter of 19 mm.The loading of SnO 2 @MX in modified PP was about 0.1 mg cm −2 .The MX-PP was processed using the same method, but with SnO 2 @MX replaced by MXene.More details of other syntheses and characterizations can be seen in Supporting Information.

Theoretical Calculation
To understand the roles of the Mott-Schottky heterostructure and its adsorption/catalytic mechanism in our work, we provide a schematic of the formed SnO 2 @MX heterojunction.As depicted in Fig. 1a and b, when metallic Ti 3 C 2 T x MXene contacts semiconducting SnO 2 , electron transfer from SnO 2 to MXene occurs to equilibrate the respective Fermi levels.Note that the work function of Ti 3 C 2 T x MXene is higher than that of SnO 2 according to the literature [43,44].This electron transfer results in the formation of a depletion region carrying a positive charge on the SnO 2 side and an accumulation region with a negative charge on the MXene side, leading to the generation of an internal electric field, commonly known as built-in electric field (BIEF).Figure S1 shows the direction of the electric field on the surface of the SnO 2 @MX heterostructure.Significantly, the electron coordination environment in both of these regions changes from the initial state, impacting the electronic structure and influencing the capability of surface adsorption as well as the catalytic activity toward LiPSs.
Density functional theory (DFT) calculations were performed to explore the supporting role of SnO 2 on Ti 3 C 2 T x (T=O or OH as representative) along with the adsorption behavior toward the Li 2 S 6 cluster.Figures 1c and S2 reveal the pronounced interfacial effect of the SnO 2 slab on the surface charge modulation.In the presence of SnO 2 , charge localization slightly reduces, aligning with the enhanced negative adsorption energy values (Table S1).This underscores the instrumental role of SnO 2 slab in facilitating adsorption of the Li 2 S 6 cluster.Partial density of states (PDOS) of d-band and non-metal p-band were further analyzed to elucidate changes in the band structure brought about by the SnO 2 slab (Fig. 1d and Table S2).Upon integration with SnO 2 , both the d-band and non-metal p-band centers upshift with respect to the Fermi level.Moreover, the energy gaps between d-and anion p-band center are reduced, which indicates reduced energy gaps between bonding and antibonding orbitals, thereby facilitating the electron transfer and consequently enhancing the conversion of LiPSs.Moreover, a further analysis of bonding and antibonding states was conducted using the crystal orbital overlap population (COOP), as shown in Fig. S3.The SnO 2 -supported MXene exhibited enhanced bonding states, indicated by a positive sign in COOP and a negative sign in crystal orbital Hamilton population (COHP), compared to its counterpart without the SnO 2 slab.This observation aligns well with the calculated adsorption energy.Above results demonstrate that the formation of a heterostructure on the MXene plane can regulate the adsorption strength and facilitate the catalytic activity, offering significant promise in promoting the reversible transformation of LiPSs.

Characterization of SnO 2 @MX Heterostructure
Motivated by above theoretical calculations, a 0D-2D Mott-Schottky SnO 2 @MX heterostructure was fabricated.As shown in Fig. 2a, the delaminated Ti 3 C 2 T x MXene was first synthesized via the MILD route based on commercial Ti 3 AlC 2 powder.After a hydrothermal procedure, the SnO 2 @MX heterostructure with nucleated SnO 2 seeds decorating the MXene nanosheets (named as SnO 2 @MX) was obtained.The ultrathin MXene nanosheets appear almost transparent under the electron beam (Fig. S4), and SnO 2 is distinctly visible and generally dispersed across the MXene flakes (Fig. 2b).We also observed instances of localized aggregation, suggesting that the dispersion, although predominantly homogeneous, includes areas where SnO 2 particles have clustered (Fig. S5).The distribution is further confirmed by energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Fig. 2c).The high-resolution transmission electron microscope (HRTEM) analysis reveals the presence of rutile SnO 2 QDs, as evidenced by the crystal spacing and diffraction rings of (110), (101), and (211) planes (Fig. 2d).The average size of the QDs is ~ 3.9 nm (Fig. S6).Such small particles ensure numerous It is interesting to note that, compared to MXene, the structural morphology of SnO 2 @MX remains largely unchanged (Fig. S7), which enables the fabrication of a compact, low-porosity laminar structure suitable as a separator interlayer in Li-S batteries.As shown in Fig. S8, SnO 2 @ MX slurries can be directly cast onto the PP separator using an industrially compatible doctor-blade technique, with no additional binders required.The as-prepared SnO 2 @MXmodified PP (SnO 2 @MX-PP) was characterized by scanning electron microscopy (SEM).Figure S9 shows MXene and SnO 2 @MX covering the porous PP separator.It is worth noting that the MXene-modified PP (MX-PP) exhibits distinct grooved stripes, which may be attributed to the different surface properties of the two materials.Atomic force microscopy (AFM) characterization reveals that the surface roughness of the SnO 2 @MX interlayer is significantly higher than that of the MXene interlayer, which suggests a substantial number of nanoparticles decorating the MXene plane (Fig. 2e).In fact, SnO 2 @MX shows better adhesion to the PP separator (compared to MXene) with remarkable structural and mechanical robustness (Fig. S10).The crosssectional SEM image reveals the dense parallel stacking of SnO 2 @MX nanosheets, with a thickness of 543 ± 54 nm (Fig. 2f).Such ultrathin thickness and negligible mass effectively minimize its impact on the cell energy density.Additionally, the wettability of the separator interlayer was also validated using contact angle (CA) measurements.In contrast with the relatively stable CA (≈ 45°) formed between PP separator and electrolyte, the electrolyte rapidly spreads on both SnO 2 @MX-PP and MX-PP, resulting in a CA close to 0° (Figs.2g and S11).This indicates excellent wettability of both SnO 2 @MX and MXene separators by the electrolyte, which guarantees efficient interactions with LiPSs and is beneficial for Li ion transportation.
To explore the chemical states of SnO 2 @MX, including variations with different Sn concentrations, we synthesized the SnO 2 @MX samples with varying amount of Sn precursors: SnO 2 @MX, 0.5-SnO 2 @MX, and 2-SnO 2 @MX (refer to the experimental section for details).Additionally, a synthesis omitting Sn precursor was conducted, resulting in a variant denoted as H-MX to further our comparative analysis.X-ray photoelectron spectroscopy (XPS) analysis was performed to gain insight into the valance states of Sn and Ti.In the Sn 3d spectra, doublet peaks at 487.4 and 495.8 eV correspond to the 3d 5/2 and 3d 3/2 orbitals of SnO 2 , respectively (Fig. 3a).The peak intensities increase with more Sn precursor used.The Ti 2p spectra for Ti 3 C 2 T x exhibit doublets at 455.6/461.6,456.4/462.4,and 457.6/463.6 eV, with a 6.0-eV splitting energy.And the doublet at 459.6/465.3eV is attributed to TiO 2 .Even though the TiO 2 peaks in the SnO 2 @MX samples are higher than in pristine MXene, they are noticeably smaller than the TiO 2 peaks emerging in H-MX, indicative of MXene oxidation during the hydrothermal process (Fig. S12).In the SnO 2 @MX samples, the higher Sn precursor concentration, the smaller the TiO 2 peak intensity.This can be ascribed to the tight decoration of the SnO 2 seeds, which isolates MXene flakes from direct exposure to aqueous solution [45].The TEM image shows TiO 2 nanocrystal rods (~ 50 nm in length) in H-MX (Fig. S13).Nevertheless, there are no distinct TiO 2 observed in the XRD of H-MX and SnO 2 @MX samples, which could be attributed to the limited oxide content (Figs.3c and S14).In MXene, we do observe the characteristic peak of MXene at 2θ = 7.2° for (002).Interestingly, both H-MX and SnO 2 @ MX samples exhibit a shift of this peak toward lower angles, with 2θ values of 6.3° and 6.6°, respectively.According to Bragg's law, this shift toward lower 2θ angles implies an increase in the d-spacing, indicating expanded interlayer spacing (Figs.S15 and S16).Such expansion is beneficial for Li + transportation in Li-S batteries, as will be discussed below.

Electrocatalytic Optimization of SnO 2 @MXs and Adsorption Behavior Toward LiPSs
To optimize the SnO 2 content in SnO 2 @MX for maximum catalytic activity and investigate the catalytic effect of TiO 2 derived from MXene, kinetics evaluations were further conducted on the prepared samples.Li 2 S 6 symmetric cells were assembled to assess the transformation kinetics of liquid-phase LiPSs.The CV curves in Fig. 3d 3g illustrates the role of SnO 2 @MX catalyst in Li-S cells.The well-constructed heterostructure enriches the electrochemical active sites at phase boundaries, lowering Li 2 S nucleation barriers, boosting charge transfer and conversion kinetics, and ensuring the re-utilization of LiPSs.
To investigate the LiPSs adsorption behavior, an adsorption test was performed using a Li 2 S 6 solution.As shown in Fig. 3h, the Li 2 S 6 solution immersed with SnO 2 @MX turned colorless after standing still for 24 h, whereas the LiPS solution with an equal amount of MXene still exhibited a light yellow color.UV-Vis spectra further confirmed the absence of LiPS in the supernatant after immersion, as evidenced by the weakened peaks at 260 and 280 nm.The SnO 2 @MX-Li 2 S 6 group demonstrated the lowest peak intensity as well as the lightest solution Fig. 3 XPS analyses of a Sn 3d and b Ti 2p in 0.5-SnO 2 @MX, SnO 2 @MX, and 2-SnO 2 @MX (these three samples are collectively termed SnO 2 @MXs in the following).c XRD patterns of MXene and SnO 2 @MXs.Electrocatalytic activity tests of d CV curves (up) of symmetric cells with Li 2 S 6 dissolved in electrolyte as active material and the corresponding Tafel plots (down) with the scan rate of 10 mV s −1 .Potentiostatic e precipitation and f dissolution tests of SnO 2 @MXs electrodes with Li 2 S 8 dissolved in the electrolyte.g Schematic illustration of the efficient LiPSs adsorption and catalytic conversion process in SnO 2 @MX heterostrctures, where the 2D flakes are MXene (green color), LiPSs chains are adsorbed on SnO 2 (sphere with white color) surface.h Adsorption test using UV-Vis spectra of Li 2 S 6 in DOL and after adsorption by MXene and SnO 2 @MX powder, with inset showing a digital image of the solution from above after equilibration for 24 h.The solution from left to right numbered one to three, is, respectively, Li 2 S 6 , MXene-Li 2 S 6 , and SnO 2 @MX-Li 2 S 6 solution color, indicating the strongest adsorption capability of SnO 2 @MX toward LiPSs.To visualize the inhibition of LiPSs migration through the separator, diffusion tests were performed using a H-typed glass apparatus (Fig. S19).It was observed that both the PP and MX-PP separators showed obvious LiPSs penetration after 24 h.However, the migration of LiPSs across the SnO 2 @MX-PP separator was effectively restrained.Therefore, the SnO 2 @MX-PP separator demonstrates superior inhibition of the diffusion of LiPSs, thus contributing to significant suppression of the shuttle effect in Li-S batteries.

Comparison of Electrochemical Performance
To evaluate the potential application of the optimized SnO 2 @ MX interlayer in Li-S batteries, the electrochemical behaviors of a series of coin cells were tested with CNT (carbon nanotubes)/S as cathode and SnO 2 @MX-modified PP as separator (Fig. S20). Figure 4a shows the cyclic voltammetry (CV) curves of cells with PP, MX-PP, and SnO 2 @MX-PP separators at a scan rate of 0.1 mV s −1 , respectively.Two distinct cathodic peaks located at 2.2 ~ 2.3 and 1.9 ~ 2.0 V correspond to the liquid-liquid phase transformation and liquid-solid phase transition of sulfur species.Conversely, The cells were tested after 1 cycle of activation process).h GITT plots of Li-S cell with SnO 2 @MX-PP separator and i the derived internal resistance plots of cells with the three separators with respect to normalized time.j CV curves of the cell with SnO 2 @MX-PP separator at gradient scan rates and k the plots of the peak 2 current versus the square root of scan rates 1 3 the oxidation process exhibits overlapping peaks, resulting in an anodic peak located at 2.3 ~ 2.4 V. Notably, the cell with SnO 2 @MX-PP separator demonstrates significantly higher peak current and lower voltage polarization, indicative of improved conversion kinetics in the Li-S cell.The galvanostatic charge-discharge (GCD) profiles at 0.05 °C are displayed in Figs.4b and S21.The two discharge plateaus and one charge plateau in the curves are consistent with the CV peaks.Remarkably, the cell with SnO 2 @MX-PP separator demonstrates a significantly higher initial discharge capacity of 1414 mAh g −1 , surpassing both PP-and MX-PP-based Li-S cells.The specific capacities for the first and second plateau, denoted as ΔQ 1 and ΔQ 2 , respectively, are illustrated in Fig. 4c.We find SnO 2 @MX-PP displays a capacity boost in both plateaus, with more pronounced increase in the second plateau (25.3%) than the first (12.1%).Electrochemical impedance spectroscopy (EIS) measurements show a smaller charge-transfer impedance (R ct ) for SnO 2 @MX-PP compared to MX-PP and PP cells (Fig. 4d and Table S3), indicating superior redox kinetics in the SnO 2 @MX-PP system.
Rate capabilities of cells were compared under a stepwise increase in current rates from 0.1 to 2 °C (Fig. 4e).The cell with SnO 2 @MX-PP separator consistently exhibits superior capacity at each gradient of current rate.Even at a high current rate of 2 °C, a satisfactory capacity of 845 mAh g −1 is obtained.After cycling at varying rates and returning to 0.1 °C, the capacity recovers to 1193 mAh g −1 (30th cycle), nearly matching the initial 1198 mAh g −1 (5th cycle).Figure 4f shows the cyclic performance evaluated at a small current rate of 0.05 °C.After activation, the SnO 2 @MX-PP outperforms both MX-PP and PP, maintaining 991 mAh g −1 after 50 cycles, equivalent to 79% of capacity retention, compared to 76% for MX-PP and 68% for PP, respectively.This underscores the SnO 2 @MX interlayer's efficiency in enhancing sulfur utilization and reducing capacity decay by suppressing LiPSs shuttling and promoting their conversion.The long-term cyclic performance was also studied under a high current rate of 2 °C (Figs.4g and S22).The SnO 2 @MX-PP delivers an initial discharge capacity of 844 mAh g −1 , which reduces to 625 mAh g −1 after 500 cycles (0.052% decay per cycle), while MX-PP and PP start at 760 and 624 mAh g −1 , decaying at 0.069% and 0.084% per cycle, respectively.This highlights the superior capacity and enhanced long-term stability of the SnO 2 @MX-PP in Li-S batteries.After undergoing and extensive aging within the cell, the SnO 2 @MX-PP is extracted and further analyzed, demonstrating a decent stability of SnO 2 @ MX composite (Fig. S23).
Galvanostatic intermittent titration technique (GITT) measurements were applied to examine the internal resistances of the cells employing different separators (Figs.4h and S24).The polarization occurring during electrochemical operation is quantified by determining the internal resistance ( ΔR inter ) using the following equation: where ΔV QOCV−CCV represents the voltage difference between the quasi open-circuit voltage (QOCV) and closedcircuit voltage (CCV), and I appli is the applied current.The values of ΔR inter are plotted in Fig. 4i for the three cells based on different separators, as function of normalized time for both charge and discharge processes.It is interesting to find that the ΔR inter in Li 2 S formation and dissolution regimes is significantly higher than in LiPSs conversion regimes, indicating Li 2 S-related reactions encounter the highest energy barrier during charge and discharge.Moreover, the SnO 2 @ MX-PP demonstrates smaller ΔR inter values throughout the entire process, suggesting lower internal resistance and enhanced reaction kinetics than the other cells.Li + diffusivity was analyzed using sequential CV measurement on Li-S cells at various scan rates (Figs.4j and S25), identifying two cathodic and one anodic peak as peaks 1, 2, and 3.The relationship between the peak current ( I p ) and scan rate ( ν ) can be described by the Randles-Sevcik equation [47]: where D Li is Li + diffusion coefficient, z is the number of transferred charges, A is the surface area of the electrode, and c is the Li + concentration.The peak current ( I p ) exhibits a linear relationship with 0.5 , where the slope is associated with D 0. 5  Li .As shown in Figs.4k and S17, the SnO 2 @ MX-PP displays a higher slope compared to MX-PP and PP, with calculated Li + diffusion coefficients of 2.97 × 10 -7 , 4.70 × 10 -7 , and 1.52 × 10 -6 cm 2 s −1 for peaks 1, 2, and 3, respectively.These results suggest that incorporating the SnO 2 @MX-PP interlayer effectively boosts the Li + diffusion rate and promotes the kinetics of the LiPSs redox process.

Lithium Dendrite Growth Suppression by SnO 2 @ MX
Since Li dendrite growth in the Li anode poses safety risks to pierce the separator and cause short circuits, the introduction of SnO 2 @MX is believed to be beneficial in suppressing Li dendrite formation due to the high Young's modulus, efficient Li + conduction ability, and lithophilic sites (Fig. 5a and  b) [48].To validate this, symmetric Li/Li cells were assembled and subjected to galvanostatic cycling to assess the lithium plating/stripping behaviors.Figure 5c shows the voltage profiles of the symmetric cells at different current densities of 1, 2, 3, and 5 mA cm −2 .Notably, the SnO 2 @MX-PP Li/ Li cell exhibits superior rate performance with consistently lower overpotential than the PP-based cell.During long-term cycling at 1 mA cm −2 (Fig. 5d), the SnO 2 @MX symmetric cell exhibits superior stability over 200 h, with consistent voltage hysteresis and only about 20-mV polarization.Meanwhile, the PP-based cell displays fluctuating overpotentials (> 20 mV), indicating significant polarization and unstable solid electrolyte interface (SEI) formation.Li/Cu cells were also constructed to assess nucleation overpotentials.As shown in Fig. 5e, the SnO 2 @MX electrode exhibits a reduced nucleation overpotential (61 mV) compared to that of bare Cu electrode (86 mV), underscoring its capability to guide uniform Li plating by reversing dendrite growth.This is further confirmed by the SEM images in Figs.5f and S26, as a smooth, flat surface is found in cycled Li electrode from the SnO 2 @MX-PP-based cell while a rough and patchy surface is observed in the PP-based cell.Thus, the SnO 2 @ MX interlayer not only exerts an advantageous effect on the cathode but also demonstrates the ability of inhibiting Li dendrite formation on the anode side.

Evaluation of High Sulfur Loading Performance
Considering the practical application of Li-S batteries, the performance of cells with high sulfur loading and SnO 2 @ MX-PP separator was further investigated.The SnO 2 @ MX heterostructure interlayer was coated on both sides of the PP separator, providing functions to suppress shuttling and catalyze the conversion of LiPSs, as well as inhibiting Li dendrite growth on the other side (Fig. 6a). Figure 6b shows the initial charge-discharge profiles of SnO 2 @ MX-PP-, MX-PP-, and PP-based Li-S cells under a high sulfur loading of 3.8-4.0mg cm −2 .A voltage valley before the second discharge plateau in the PP-based cell indicates a high-energy barrier for Li 2 S nucleation.The overcharging observed during the charging process also suggests a severe shuttle effect under high sulfur loading conditions [49].In contrast, the SnO 2 @MX-PP-based cell exhibits facilitated Li 2 S nucleation and inhibition of LiPSs shuttling.The presence of highly catalytic heterojunction sites significantly lowers the nucleation energy, enabling more efficient nucleation without noticeable barrier.Figure 6c displays the cyclic performance of SnO 2 @MX-PP at various rates.The cell achieves an initial capacity of ~ 1160 mAh g −1 at 0.1 °C and maintained stability over 50 cycles, even at higher current rates of 0.2 and 0.5 °C.However, increasing the sulfur loading from 3.8 to 5.2 mg cm −2 led to higher overpotential and reduced area capacity from ~ 4 to 3 mAh cm −2 (Fig. 6d).Nevertheless, a decent capacity of 4.7 mAh cm −2 can still be achieved at lower current rate of 0.05 °C in a high sulfur loading of 5.7 mg cm −2 (Fig. S27).Furthermore, when the sulfur loading was increased up to 7.5 mg cm −2 , the cell realized an initial area capacity of over 7.6 mAh cm −2 and maintained stability at a 0.02 °C current rate (Fig. 6e).This higher stability over 50 cycles compared to that shown in Fig. 4f could be attributed to the lower depth of discharge levels.Nevertheless, challenges persist, including uneven electrolyte wetting and increased polarization at lower E/S ratios (Fig. S28).Table S4 presents the comparison of electrochemical performance with other recent reported works.All in all, the SnO 2 @MX-modified PP separator effectively reduces Li 2 S nucleation overpotential, inhibits Li dendrite growth as well as the shuttle effect, thereby enabling decent performance with high sulfur loadings.However, achieving high capacity with low overpotentials remains challenging when considering both high sulfur loading and current rates.Separator modification alone may not fully meet the requirement for commercialization, and it is more promising to combine novel separators like the ones developed in this work with other advancements in cathode, electrolyte, and anode technologies.Still, developing an ultrathin and efficient separator interlayer remains crucial.

Conclusion
In summary, we have developed a dense, ultrathin, and laminar SnO 2 @MX heterostructure separator interlayer to effectively suppress LiPSs shuttling, catalyze redox conversions, and inhibit Li dendrite growth in Li-S batteries.Specifically, SnO 2 QDs were integrated onto the MXene basal plane, creating boundary sites with coordination environments that enhance LiPSs immobilization and rapid charge transfer.The synergistic effects of SnO 2 , MXene, and their heterojunctions effectively modulate the reaction kinetics, encompassing LiPSs trapping, diffusion to boundaries, Li 2 S nucleation, growth, and dissolution.Thanks to the unique properties of the heterostructure and the structural advantage of the SnO 2 @MX interlayer, the SnO 2 @MX-PP Li-S cell exhibits superior electrochemical performances.It demonstrates a high area capacity of 7.6 mAh cm −2 at a high sulfur loading of 7.5 mg cm −2 , exceptional rate capability with a capacity of 845 mAh g −1 at 2 °C, and remarkably cyclic stability with a capacity fading of only 0.052% per cycle over 500 cycles.Our work not only demonstrates a feasible strategy of utilizing a laminar separator interlayer for advanced commercialized Li-S batteries, but also provides valuable insights into the understanding of heterostructure catalysis and its role in boosting catalytic reaction kinetics.

Fig. 1
Fig. 1 Energy band diagram of Mott-Schottky type contact between MXene (work function W m = 4.37 eV) and SnO 2 (work function W s = 3.84 eV) a before and b after contacting.c Electron localization functions and the Li 2 S 6 adsorption energy on the -O-terminated MXene (abbreviated MXO in the figure), SnO 2 -supported -O-terminated MXene (abbreviated MXO-SnO 2 in the figure), -OH-terminated MXene (abbreviated MXOH in the figure), and SnO 2 -supported -OH-terminated MXene (abbreviated MXOH-SnO 2 in the figure), from left to right.The color presents the degree of localization of electrons, e.g., blue (localization) to red (delocalization).The surface level is set to 0.6.d Partial density of states (PDOS) analysis of metal d and non-metal p bands in both MXO and MXOH, with or without SnO 2 support.The energy gaps between p-and d-band center for MXO, MXO-SnO 2 , MXOH, and MXOH-SnO 2 are 4.18, 2.92, 4.41, and 4.04 eV

Fig. 2 a
Fig. 2 a Schematic diagram of the synthesis process of SnO 2 @MX.b TEM image of SnO 2 @MX.c EDS elemental mapping images of SnO 2 @ MX. d HRTEM image with fast Fourier transform (FFT) pattern of SnO 2 @MX.e AFM image of MX and SnO 2 @MX interlayer surface.f Cross-section SEM image of SnO 2 @MX-PP separator.Contact angles between electrolyte and g PP and h SnO 2 @MX-PP separator

Fig. 4 a
Fig. 4 a CV curves of the Li-S coin cells with various separators (PP, MX-PP, and SnO 2 @MX-PP) at a scan rate of 0.1 mV s −1 .b Charge/discharge profiles of cells with various separators and c the corresponding comparison of the first discharge plateau (ΔQ 1 ) and the second discharge plateau (ΔQ 2 ) capacities.d Nyquist plots of Li-S cells with various separators from EIS tests before cycling.e Rate performance and cyclic performances at f 0.05 C and g 1 C of Li-S cells with various separators (In the graph, solid markers represent discharge, and hollow markers represent charging.The cells were tested after 1 cycle of activation process).h GITT plots of Li-S cell with SnO 2 @MX-PP separator and i the derived internal resistance plots of cells with the three separators with respect to normalized time.j CV curves of the cell with SnO 2 @MX-PP separator at gradient scan rates and k the plots of the peak 2 current versus the square root of scan rates

Fig. 5 a
Fig. 5 a, b Schematic diagram of inhibition of lithium dendrite growth.c Rate performance and d cyclic performance of Li/Li symmetric cells with PP or SnO 2 @MX-PP separator.e Voltage-capacity profile of lithium plating and stripping in Li/Cu cell.f SEM of pristine Li and Li electrode surface with various separators after 400 cycles

Fig. 6 a
Fig. 6 a Schematic diagram of the function of SnO 2 @MX coating on both cathode and anode sides.Electrochemical performances of b chargedischarge curves with PP, MX-PP, and SnO 2 @MX-PP separator at 0.05 °C with sulfur loading of 3.8-4.0mg cm −2 and E/S (electrolyte/sulfur) ratio of 20 μL mg −1 .c Cyclic performance of SnO 2 @MX-PP at different rates with a sulfur loading of 3.8 mg cm −2 and E/S ratio of 20 μL mg −1 .d Charge-discharge curves of the first (solid lines) and 10th (dot lines) cycle with CNT various sulfur loadings at 0.1 °C.e Cyclic performance of SnO 2 @MX-PP at 0.02 °C with a sulfur loading of 7.5 mg cm −2 and E/S ratio of 10 μL mg −1